Fix error handling bugs

So we were sending some bad requests to Clever that then hit some error handling code in clever-ruby, which exposed that there are some bugs in the error handling code.

One I introduced when I changed `@@api_base` to be replaced by `configuration.api_base` but `@@api_base` was still being referenced in an error message. I'm sorry; I should have searched and made sure I caught all of those. I don't see any remaining.

The other issue is not all of the error classes were being required in the main clever-ruby file.

Thanks and sorry again!!!